As is known, flexographic printing machines with central drums have a plurality of printing plate rollers and anilox rollers arranged around a central drum; the printing plate rollers and anilox rollers transfer the ink onto the material to be printed depending on the printing pressures, i.e., depending on the pressure applied by each printing plate roller against the central drum and by each anilox roller against the respective printing plate. The pressure depends on the relative position of the printing plate with respect to the central drum and on the relative position of the anilox with respect to the printing plate.
Job changing, which occurs whenever the elements that cooperate to perform the printing process change, which elements in flexographic printing are the printing plate, the printing plate double adhesive, the printing plate sleeve, the anilox, the material to be printed and the inks, as well as the variation in the environmental conditions, such as temperature and humidity, always requires, for obtaining the desired print, the identification of the correct printing pressures, i.e., of the correct relative positions between the printing plate and the drum and between the anilox and the printing plate with respect to those calculated by the machine on the basis of the theoretical dimensions of the printing elements.
Identification of the correct pressures entails the production of product rejects and a waste of time and depends greatly on the skill and experience of the printer.
Today there are already automatic systems that aid or replace the printer in identifying and applying the correct printing pressures at the beginning of the printing process after job changing.
These systems can be divided substantially into two types: systems with print feedback; and offline systems without print feedback.
The former substantially measure printed markings (the markings are representative marks of the print) or the entire print with a video camera and then correct the printing pressures (see for example EP 1249346 and EP 1666252).
The latter, substantially before printing, measure the surface dimensions of the printing plate roller with systems which are external to the machine, store the measurement, then mount the printing plate roller on the machine and position it in contact with the central drum according to the stored data (see for example US 2008/0141886 A1).
However, these systems have some problems.
First of all, all systems with print feedback use a video camera that is capable of measuring and processing the entire printed image or more frequently specific markings that are representative of the printed image; the presence of these markings requires new printing plates and does not allow the use of old ones; moreover, each system uses different markings. The markings are located on the sides of the material and for an equal size of the printed image they require wider material and therefore a waste of material.
Furthermore, all “online” systems with print feedback control the movement of the printing plate rollers and of the anilox rollers so as to vary the printing pressures, wait for the print to be read by the video camera, measure the effect of the motion of the printing plate rollers and anilox rollers on the print and resume this process until the pressures are found to be correct; this operation takes a long time and produces considerable waste, which increases as the distance of the video cameras from the printing of the colors increases, as generally occurs due to space occupation problems.
All “online” systems with print feedback operate by comparing the print or the printed markings with a reference image that is representative of the print that one wishes to obtain in order to determine the correct printing pressures; the printer must therefore load this reference image into the system for example by means of a file.
All offline systems without print feedback measure only the dimensions of the printing plate rollers and anilox rollers when they are not mounted on the machine; they do not measure the other elements that are responsible inside the machine for the printing pressures, i.e., the printing material, the inks, the tolerances and the mechanical plays; these and the relationships among them change after a change of job and can change over time with an effect on printing pressures.
A further problem is that all offline systems without print feedback do not measure the print, are systems without feedback and therefore less precise and often require the intervention of the printer, who must retouch the printing pressures in order to obtain the desired print.
Last but not least, all these systems are complicated and expensive, produce a considerable waste of product, require time for setting up the new job for printing and almost always require the intervention of the printer, who has to retouch the printing pressures in order to obtain the desired print.